The monitoring of fluid pressure within a body organ provides an important tool for medical research and clinical diagnosis. For example, hydrocephalus and head injuries can cause body fluids to build up within the brain. The resulting fluid pressure buildup can result in death or serious brain damage. In another example, urinary dysfunction can cause fluid pressure to build up in the bladder. In a further example, intrapleural pressure measurements can be used to monitor the respiration of infants who have been identified as being at risk for sudden infant death syndrome.
Blood pressure measurements are particularly important for medical research and diagnosis for a variety of reasons. Such measurements provide researchers with insight into the physiology and functioning of the heart. Blood pressure measurements also provide researchers with useful information regarding the safety and efficacy of pharmaceuticals and the toxicity of chemicals. By transducing blood pressure into a signal waveform, a variety of useful parameters can be extracted. These parameters provide valuable information for the diagnosis of heart disease. Left ventricular (LV) blood pressures measurements are particularly important because the left ventricle chamber of the heart pumps blood to the systemic circulatory system, that is, throughout the rest of the body.
Common parameters extracted from left ventricular blood pressure waveforms include peak systolic pressure (the high pressure peak resulting from a contraction of the left ventricle chamber of the heart), end diastolic pressure (the low pressure valley resulting from expansion of the left ventricle), and maximum dP/dt (a peak value of how fast the pressure (P) changes with time (t) during a contraction of the left ventricle). These blood pressure measurements provide helpful diagnostic information to the physician.
For example, maximum dP/dt provides a measure of the work that is being done by the heart. For certain conditions, such as congestive heart failure (CHF), it is desired to reduce the work load on the heart. The treating physician can determine how effective a therapy is by determining if the treatment regimen has indeed reduced the work load on the heart, as indicated by the maximum dP/dt signal extracted from the left ventricular blood pressure waveform. Measurement of left ventricular blood pressure is also useful for titrating new drugs for treating heart disease, that is, determining the desired dosage or concentration of a new drug. Titrating new drugs requires information on how these drugs are affecting the heart.
For example, beta adrenergic blocking drugs are often effective at treating arrhythmias and improving patient hemodynamics. However, such drugs are difficult to titrate. Because left ventricular blood pressure parameters, such as maximum dP/dt, provide information on how the heart is functioning, monitoring these parameters allows a physician to more easily determine the most appropriate dose of the drug for treating the patient. The maximum dP/dt signal, if available, could also be used as a feedback mechanism in a system that automatically delivers therapy to adjust the work load of the heart. The delivery of therapy is automatically adjusted based on the work load of the heart, as indicated by the maximum dP/dt signal.
In another example, left ventricular blood pressure provides useful information for controlling a cardiac rhythm management system. Cardiac rhythm management systems include, among other things, pacemakers, of pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart. Heart contractions are initiated in response to such pace pulses. By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One example of using a cardiac rhythm management device to control heart rate in proportion to left ventricular blood pressure is described in Mehra U.S. Pat. No. 5,129,394. The '394 patent, however, does not disclose sensing actual left ventricular blood pressure. Instead, it discloses a pressure sensor located in the coronary vein. The coronary vein extends from the right atrium through the heart tissue near the left ventricle. Because of its small size, the coronary vein is difficult to access for inserting a pressure sensor. Moreover, blood pressure sensing in the coronary vein provides only an indirect approximation of the actual left ventricular blood pressure.
Other existing techniques for monitoring left ventricular blood pressure also have drawbacks. One technique of measuring left ventricular blood pressure is described in Brockway et al. U.S. Pat. No. 4,846,191, which is assigned to the assignee of the present application. The '191 patent describes a pressure sensor that is implanted in the abdomen of a laboratory animal. The pressure sensor is connected to an organ, such as the heart or the brain, via a fluid-filled pressure transmitting catheter (PTC). One limitation of this device is that it requires invasive access to the organ in which fluid pressure is to be monitored.
For example, in monitoring left ventricular pressure, one surgical technique for using the device described in the '191 patent involves performing a highly invasive laparotomy procedure. In this procedure, the pressure transmitting catheter is passed through an incision in the diaphragm and an incision into the apex (bottom tip) of the heart. The high blood pressure in the left ventricle further increases the risk of making such incisions directly into the left ventricle. This procedure typically requires a two week recovery period for the laboratory animal. Moreover, because catheterization of the apex involves significant risks, this technique would likely be considered too invasive for human use.
Alternatively, an incision may be made into the aorta, which is the primary artery carrying blood from the left ventricle to the rest of the body. The pressure transmitting catheter is then passed into the aortic incision for measuring blood pressure in the aorta. Aortic incisions are also problematic because of the high blood pressure in the arterial circulatory system. Moreover, measuring blood pressure in the aorta does not provide a direct measurement of blood pressure in the left ventricle; such information is unavailable, for example, when the aortic valve is closed. Alternatively, the pressure transmitting catheter could be passed through the aortic valve into the left ventricle. However, leaving the pressure transmitting catheter extending through the aortic valve for a long period of time risks damage to the aortic valve as a result of the high blood pressure in the left ventricle. Thus, this procedure is also likely unsuitable for human use, particularly for chronic left ventricular blood pressure monitoring, i.e., monitoring over an extended period of time.
Another technique for measuring left ventricular blood pressure is described in Pohndorf et al. U.S. Pat. No. 5,353,800. A distal end of a pressure sensing lead is transvenously introduced into the right ventricle of the patient's heart. A hollow needle at the distal end of the lead is punched through the ventricular septum, that is, through the wall separating the right and left ventricles. This provides access to the left ventricle for sensing pressure gradients that are communicated through the hollow needle to a pressure sensor that is outside of the left ventricle. Because this procedure involves invasively forming an opening in the septum, it creates significant risks for human cardiac patients who are likely already very sick and vulnerable to such risks.
A further technique for measuring left ventricular blood pressure uses a pressure sensing catheter, such as a “Millar catheter,” available from Millar Instruments, Inc., of Houston, Tex. The pressure sensing catheter is passed through the left atrium and through the mitral valve (which separates the left atrium and left ventricle) into the left ventricle. As discussed above, however, high blood pressures exist in the left ventricle, which would likely result in damage to the mitral valve if the catheter were left interposed in the mitral valve for a long period of time. As a result, if a sequence of successive measurements is to be obtained over a long period of time, the patient must undergo recatheterization for each measurement. However, catheterization itself involves risk, discomfort, and expense, making multiple catheterizations of the patient very undesirable.
In summary, present techniques for measuring left ventricular pressure are too invasive for human use and unsuitable for use over an extended period of time. Physicians and researchers need less invasive techniques for chronic measurement of left ventricular blood pressure, both for diagnosing heart conditions and for determining whether therapy delivered to the heart is adequate for effectively treating the patient's symptoms.